This invention relates to internal combustion engines designed to improve efficiency, improve power to weight ratios, and reduce emitted pollutants in a configuration which is readily manufacturable. The invention is most applicable to engines used in automotive applications.
A major objective of the invention is to provide a prime mover engine, i.e. a device to derive mechanical energy from the heat energy of a burning fuel, with higher efficiency in a lighter weight and smaller configuration than has heretofore been the case; particularly at power demands less than the engine""s maximum. The main use for the invention is for automobile power: For this application efficiency at low engine torque at moderate speeds is of prime interest since most of the time an automobile engine operates at approximately 10% of its maximum power output at moderate speeds-typically 1,500 to 3,000 rpm.
The engineering terminology used in this specification follows standard mechanical engineering practice. Three works have been used as engineering reference. These are:
Avallone and Baumeister, Ed., Marks"" Standard Handbook for Mechanical Engineers, Tenth Edition, McGraw-Hill, 1996: referred to as xe2x80x98Marksxe2x80x99.
Ricardo, Harry R., The High Speed Internal Combustion Engine, Fourth Edition, Blackie and Son, Ltd., 1967: referred to as xe2x80x98Ricardoxe2x80x99.
Stephenson, R. Rhoada., Should We Have a New Engine?, Jet Propulsion Laboratory, California Institute of Technology, 1975: referred to as xe2x80x98Stephensonxe2x80x99.
Current automotive practice is usually to employ a spark-ignition engine with an average thermal efficiency around 20%; i.e. about 20% of the thermal energy of the fuel used is transferred to mechanical energy. Alternatively, a compression-ignition engine, more commonly called a diesel engine, is used having a somewhat higher efficiency at low output. The added efficiency of the diesel engine is, in passenger car application, offset by the added weight of current diesel engines. A typical passenger car using a diesel engine is no more efficient than a car of equal performance using a spark engine. Comparisons of apparent mileage differences between spark engines and diesel engines is obscured by the difference in energy content of diesel fuel and gasoline. Diesel fuel has about 18% more energy for a given volume, liter or gallon, than does gasoline: Thus an accurate comparison of a diesel car that gave 40 mpg with a spark-engine driven car giving 32 mpg would show that the two vehicles use almost exactly the same amount of energy. Even more exact comparisons that consider performance of the two autos shows that the diesel-driven car is most often less efficient than an equivalent spark-engined vehicle. Support for this argument comes from the choice of Toyota and Honda in their choice of spark engines for the Prius and Insight vehicles respectively. These two cars are designed to provide the ultimate in fuel mileage using contemporary techniques.
The discussion above begins to illustrate the problem of increasing the efficiency of automobiles. It is not enough to increase maximum efficiency of the prime mover; the efficiency at low power outputs and the weight of the engine are of equal or greater importance. In order to accomplish this increase of system efficiency it is necessary to reduce engine friction; increase engine power-to-weight, and focus on increasing the efficiency of the detailed burning process in the engine. In today""s environment it is also necessary to ensure that the engine does not pollute the environment. If the engine is not inherently clean any accessories added to remove exhaust pollutants to the degree needed today can easily reduce efficiency directly and the weight added for these accessories will detract from the vehicle""s fuel mileage.
Current proposals mostly fail to globally address the complexity of this problem. Any solution that addresses internal combustion engine efficiency needs to consider the basic combustion process itself. To obtain high efficiency at very low power outputs a solution must address the problem of lean burning. Hydrocarbon fuels do not burn rapidly enough for use in an automotive sized engine at fuel/air ratios under around 50-60% of stoichiometric ratio. To obtain ultra-efficient burning at 10% of maximum power output it is necessary to efficiently combine the fuel with air at fuel air ratios around 15-20% of stoichiometric within the time it takes an engine to rotate 30-35xc2x0 at around 2,000 rpm or about 3 milliseconds. No matter what is done to a bulk air-fuel mixture this has not proved feasible in workable systems.
Diesel engines sidestep this problem by finely dividing the fuel and spraying it into a hot air environment. The burning that results occurs around each droplet at a fuel-air ratio almost exactly stoichiometric: Thus a mixture that is nominally a bulk mixture of fuel and air at a low fuel-air ratio is really a mixture of micro-domains of fuel and air at near stoichiometric ratio. The penalties inherent in this approach include the high friction penalties attendant with the use of compression ratios around 20:1 needed for automotive-sized engines and the aforementioned added weight. This illustrates that the solution must firmly address the problem of mechanical friction.
Friction and its effect on the part-load efficiency is largely ignored in contemporary proposed automotive prime mover solutions. The effect of friction is a very complicated factor. Typical modern production automotive engines battle friction by employing sophisticated valving and induction systems to ensure that maximum bearing loads are encountered only at moderate and higher speeds, where journal bearings can endure higher pressure loadings. This allows these same journal bearings to be designed smaller and thus the bearings contribute less friction to degrade the engine""s performance.
The effect of friction is especially complicated when considered in conjunction with compression ratio. A higher compression ratio in an internal combustion engine inevitably results in concomitantly increased thermal efficiency. This is unfortunately accompanied by an increase in friction because the added compression ratio is inevitably attended by added friction from the larger bearings that are needed to support the higher loads that go along with the higher compression ratio. The friction loads are particularly influential to the engine when delivering low power at moderate speed which is the normal duty for an automotive engine.
It is highly desirable to realize an engine that is notably lighter and smaller for a given power output than conventional engines. It is well known that the fuel consumed by a road vehicle is approximately proportional to the vehicle""s weight. Combining an increase in efficiency with lowered engine weight greatly increases the fuel efficiency of a vehicle system. This is especially true when the effects of what is called, in automotive technology, weight propagation are considered. This term describes the effects of changing the weight of any component of a vehicle system. Since the component must be carried by the vehicle system and the component""s mass must be stopped by the vehicle""s brakes the inevitable effect of changing the weight of any of the vehicle""s components further entails a change in the weight of the vehicle by about 70% of the initial weight change. Thus a reduction of engine weight of 100 pounds will result in a total weight reduction of about 170 pounds due to the effects of weight propagation.
Internal Combustion Engine Pollutants
Another objective of the invention needed in today""s environment is to create a prime mover than burns fuel in a manner that is inherently clean; whose combustion process inherently produces few contaminants associated with internal combustion engines. Such an engine will need fewer or smaller cleanup mechanisms such as catalytic converters used with it to meet increasingly stringent requirements for engines in public use.
Internal combustion engine pollutants are of two general kinds: Oxides of nitrogen and unburned or partially unburned hydrocarbons (carbon monoxide production in engines can be considered as resulting from partial burning of the carbon in a hydrocarbon fuel). Diesel, or compression-ignition, engines produce particulates, microscopically small pieces of carbon and other matter due to the nature of combustion in compression-ignition engines. Well designed engines using homogeneous mixtures of fuel and air such as are burned in typical spark-ignition engines have little tendency to produce significant quantities of particulates.
Oxides of nitrogen are produced when oxygen and nitrogen are heated together to very high temperatures (ca. 2,500xc2x0 C. and above) such as occurs in burning fuel-air mixtures. Production of nitrogen oxides is intensified when burning fuel-air mixtures are close to stoichiometric ratios. Production of oxides of nitrogen is reduced in mixtures of burning fuel and air that have an excessive amount of either fuel or air and are further reduced by burning the fuel-air mixture in conjunction with inert gasses such as recycled exhaust products (EGR). Stephenson shows data from Blumberg, P., and Kummer, J. T., xe2x80x9cPredictions of NO Formation in Spark-ignited Enginesxe2x80x94An Analysis of Methods of Controlxe2x80x9d, Combustion Science and Technology, Vol. 4, pp 73-95. This showed that an engine produced vanishingly small amounts of nitrogen oxides when fuel was burned in an atmosphere with 40% excess fuel or air in surplus over stoichiometric proportions when a small amount of EGR was present. These data are shown in graphical form in FIG. 9.
Complete burning the fuel in an engine, with the consequence that small quantities of unburned hydrocarbons or carbon monoxide result from the process, is most thoroughly accomplished by burning with an excess of air over stoichiometric proportions at elevated temperatures followed by oxidation in a catalytic convertor. Thorough burning such lean mixtures, however, is not easily implemented. Uniformly mixed lean mixtures burn too slowly to be useful in an engine designed to be used at speeds of 1,000-6,000 rpm if the burning is initiated in the uniformly mixed air-fuel bulk blend.
Efficiency in Internal Combustion Engines
The efficiency of an internal combustion engine is determined by complicated relationships. In order to obtain an optimum efficiency it is necessary to balance many individual factors. Each of these tends to counteract, in some way or ways, the effects of the others. The main parameters that need to be considered in the design are:
1. Basic thermal efficiency
The efficiency of a prime mover is the percentage of heat energy obtained from the fuel burning that is converted to useful mechanical energy. Indicated thermal efficiency is a term used to describe the percentage of the energy obtained from the fuel that is converted to mechanical energy within the engine even though some of this energy may not be available outside the engine due to factors such as friction within the engine and the energy used to run ancillary mechanisms needed for engine operation. Brake thermal efficiency is the term used to describe efficiency of the engine in terms of the percentage of heat energy of the fuel that is available outside the engine as usable energy. Friction converts some of the basic mechanical energy delivered from the engine process to heat before mechanical energy is transferred outside the engine: The difference between indicated thermal efficiency and brake thermal efficiency is thus that percentage of the heat energy used up in moving engine parts against internal friction of the engine, in pressure drops undergone by gases flowing within the engine and that energy needed to drive accessory mechanisms within the engine essential to the engine""s operation. This last category includes fuel pumps, water pumps and valve gear.
2. Friction of the internal parts that occurs as engine parts move
As noted above, friction takes away from the net thermal efficiency of the engine. Mechanical friction in an internal combustion engine mostly originates from bearings supporting the crankshaft, rubbing of pistons on their cylinder walls and friction in the valve mechanism. Bearing and piston friction is dependent on loads within the engine. The loads will vary with the detailed design of the engine but are always a function of the compression ratio of the engine: A higher compression ratio results in larger bearing and piston loads. Marks, Section 8, shows that the size of bearings and their relative friction power loading is proportional to the load or force placed on the bearings. The data also show that journal bearings can support a load that is proportional to the notational speed of the bearing shaft.
The use of a large compression ratio will increase the indicated thermal efficiency of an engine. However, a rise in the compression ratio of an internal combustion engine always gives rise to an increase in the friction of a real engine, as opposed to the engine as a theoretical entity. This results in a decrease in the average operating efficiency at compression ratios over about 8 to 1 in the case of spark ignited engines used in vehicle transport. This is clearly shown in Ricardo; one of the basic texts on internal combustion engines. The relationship that leads to this conclusion is found in the fact that most of the usage of an engine for passenger road transport in particular, and practically all prime movers in general, occurs at outputs far less than the maximum that can be derived from the engine. Thus an engine that has a high efficiency at full power with a compression ratio of 10 to 1 will be less efficient in overall passenger car usage than a correctly designed engine having a compression ratio or 8 to 1 when both engines are operated at 30% of their maximum torque. This torque level is typical for passenger transportation needs and also approximately representative for many applications of prime movers. The reason for the higher efficiency of the engine using an optimum compression ratio is that the bearings and other load supporting members of the engine must be designed to be large enough to withstand the highest pressure internal to the engine that the engine will endure. This results in larger frictional losses in the engine using the higher compression ratio: These larger frictional losses are more than offset by higher indicated thermal efficiency at full torque demand but when the engine""s usage on an overall basis is analyzed the average efficiency of an engine with a compression ratio of about 8 to 1 will be more efficient than that of an engine having a compression ratio of 10 to 1. The fact that the engine is used delivering a typical torque of around 30% of maximum means that the efficiency during this service is more important to average efficiency than the efficiency of the engine delivered when the engine is used at full torque.
3. Non-linearities due to chemical interactions within the burning fuel-air mixture
A high compression ratio also incurs some chemical losses. The efficiency gains engendered by the use of higher compression ratios are obtained because heat is extracted from the fuel at higher temperatures as the compression ratio is raised: Any heat engine is more efficient as the temperature at which the heat is added to the engine is raised relative to the temperature at which heat is rejected from the engine. This comes from basic Carnot teachings. At temperatures above about 2000xc2x0 C. two effects, disassociation and non-linear specific heat, occur in the fuel-air products of carbon dioxide and water vapor; the basic products of burning organic fuels. The effect of these two phenomena is to reduce the useful amount of heat that can produce energy in the engine. Thus as an engine is designed to use higher and higher compression ratios, the deviation from theoretical efficiency increases so that the actual efficiency becomes less because of the friction effects noted previously and also due to the fact that effects of disassociation and variable specific heat counteract some of the added efficiency gained from the higher compression ratio. Chemical losses are counteracted by using lean mixtures within the engine; mixtures of fuel and air that have excessive amounts of air.
4. Pressure drops that occur as air moves into the engine and exhaust products are expelled from the engine.
As any gas passes through a tube or other like conduit a pressure gradient in the gas is required to maintain the velocity of the gas through the conduit. The same statement applies to gas passing through a port, or entrance, to the conduit or exit from such passage: A loss of pressure and thus energy is encountered wherever gas is transported at significant velocity. This energy must be supplied by the engine and thus creates a loss of efficiency. As noted above in the section on friction these pressure drops can be considered a form of mechanical friction.
Design Approaches for High Efficiency in Internal Combustion engines
Balancing the above parameters is not a simple task. The optimum engine would have negligible friction, high compression ratio, low gas velocity in all transfer passages and would burn the fuel in a lean mixture at practically all times. The VCRC engine uses a unique approach to obtain an engine close to this ideal.
The VCRC concept is based on a unique method of optimization and minimization of the losses in an internal combustion prime mover in creating a prime mover of the highest efficiency. Implementation of the concept details also results in an internal combustion engine whose combustion inherently creates little pollutants of unburned hydrocarbons, carbon monoxide or oxides of nitrogen.
System and Subsystems
Engines in accordance with the invention accomplish the above objectives by increasing the compression ratio as the torque demanded of the engine is decreased throughout the engine""s throttling range. As the compression ratio is raised the engine simultaneously provides for a leaner burning of the fuel ingested into the engine using a method of separated charge combustion. The combination of higher compression ratio together with leaner burning raises the efficiency of the engine in situations during which torque demanded of the engine is less than maximum. Since practically all applications of prime movers perform the bulk of their duties at these lower torque values the overall efficiency of systems using the inventive approach is equally increased.
This approach has many features but is characterized herein as Variable Compression Ratio and Charge (VCRC). VCRC engines particularly allow efficient throttling of two stroke cycle engines to be accomplished. This efficiency is further enhanced by a subsystem of the invention applicable to two stroke engines. With a unique arrangement of engine-driven blower and exhaust-driven turbo charger even further increases of efficiency in two-stroke versions of the VCRC can be achieved.
The VCRC engine accomplishes a reduction in both oxides of nitrogen and unburned hydrocarbons by a method of burning in two phases. First the fuel is burned in a uniformly mixed fuel-rich environment which includes some EGR. This mode of burning minimizes the creation of oxides of nitrogen. The initial burning is immediately followed by a completion of the burning process in an environment in which air is present in excessive quantities when compared with that amount needed to completely burn the fuel.
Thus in the VCRC internal combustion engine the compression ratio and the amount of fuel burned (the xe2x80x98chargexe2x80x99) during each firing cycle are simultaneously varied in response to torque demanded of the engine. A decrease in torque demand is accompanied by an increase in the engine""s compression ratio and a reduced fuel flow. The relationship of compression ratio and fuel supplied is varied in such manner as to keep the peak pressure in the engine""s combustion process nearly constant level for all torque demands at a given speed. The relationship of the two parameters of compression ratio and fuel-air ratio are also varied as speed of the engine changes so as to raise the combustion peak pressure with an increase in engine speed.
Engines in accordance with the invention also include subsystems that enable the basic engine to perform with increased efficiency and allow the design to be smaller and lighter than engines now in common usage.
The engine varies the compression ratio and mixture ratio simultaneously by arranging the engine so as to have a combustion volume in two chambers connected by a passageway. The volume of one of these chambers is varied by a separate piston subsystem mechanism: Burning is initiated in this variable volume chamber after it has been filled with a uniformly mixed fuel-air mixture. The rise in pressure and temperature caused by the initial burning forces the fuel air mixture out of the variable volume to mix with the remaining engine volume in which volume burning is completed.
The variable volume combustion chamber is varied by a piston mechanism arranged to be both reliable and easily controllable. A hydraulic snubber is used in a preferred embodiment in conjunction with a piston designed to oscillate in a reciprocating motion each engine cycle. By such design the piston remains reliably lubricated in its cylinder during operation. The hydraulic snubber provides accurate and easily implemented control of the piston""s motion.
The VCRC engine""s method of combustion offers other advantages also. By separating the combustion into two phases; an initial combustion of the bulk of fuel and air in an fuel over-rich environment followed by a completion of combustion in a high temperature fuel-lean mixture, the problems of detonation are almost entirely eliminated. Detonation, or knock as it is colloquially called, is an explosion of the last 5% or less of the bulk fuel-air mixture. An overly rapid rise in pressure brought about by the initial combustion of the fuel-air mixture creates a pressure wave that compresses an isolated mixture of fuel and air and the accompanying rise in temperature of this isolated mixture creates an explosive situation wherein this mixture spontaneously combusts giving the resultant explosive increase in pressure and noise. In the VCRC engine the xe2x80x98end gasxe2x80x99, as this isolated fuel air mixture is called in internal combustion engine engineering, consists only of air. Thus the concept of octane requirements for the fuel used are moved so far off the engine""s boundary limits as to be of essentially no import. The fuel for a VCRC engine can be most any mixture of fuel oil, of a low octane number, and gasoline with a higher value. The need for a high cetane number, necessary for smooth combustion in compression ignition engines, is equally unimportant.
The VCRC engine is exemplified here as a two-cycle engine. The invention is most suitable to the two-stroke configuration but is not limited to this: A four stroke configuration based on the same principles could also be easily realized. Some subsystems that are singularly adaptable to a two-cycle engine are also part of the invention. These include a unique method of supplying air to the engine in a manner that minimizes the losses associated with air transport.
The VCRC concept also includes a unique method of combustion to extract energy from a burning fuel-air mixture at higher efficiency than is now commercially possible in an internal combustion engine. This method of burning has the advantage of chemically combining air and fuel while creating fewer pollutants than does current engine designs. The VCRC method separates the air and fuel-air mixture in the engine into two divided volumes. Burning is initiated in the portion of the air that contains substantially all of the fuel and only part of the air used to support the combustion in a uniform mixture that is over-rich in excess fuel. The VCRC engine could be designed to provide a uniform mixture that is excessively lean as well. A perfect balance between fuel and air, called a stoichiometric ratio is avoided because this ratio results in an excessive production of nitrogen oxides. The combustion process is completed by combining the initial burned air and fuel with the remaining air. The remaining air is present in the combustion chamber in more than sufficient quantities to oxidize all the fuel in the chamber.
This method of combustion, used in conjunction with the variable volume noted above allows lean fuel-air mixtures to be burned at elevated compression ratios in an engine assembly that has low mechanical friction. This creates internal engine efficiencies higher than previously thought possible.
Stratified Charge vs. Separated Charge
Stratified charge has long been used as a method to obtain lean burning in a spark-ignition engine. There are various ramifications but most have a single generic embodiment in common. A small volume separated from the main combustion chamber is supplied with a charge of fuel and air that is rich in fuel. This charge is fired with a spark and the flame from this ignites the charge in the remainder of the combustion chamber which latter charge is much leaner. In this manner it is possible to fire charges as lean as around 50-60% of stoichiometric. Combustion that takes place only in the small separated volume is often used to support very low torque values; around 10% of maximum. Between around 10% to around 40% the typical stratified charge engine is unstable and needs other mechanisms to appropriately throttle the engine. Stratified charge design also has some problems with efficiency as well. Near the lean limit of the stratified charge approach there is trouble firing the charge in the main combustion volume rapidly enough for operation. The slow burning results in a loss of some of the heat energy of the charge and also results in incomplete combustion as well.
The VCRC engine uses what can best be termed as xe2x80x98separated chargexe2x80x99. The entire amount of fuel to be burned is contained in a separate variable volume together with around 60% or less of the air that is to be reduced by the combustion process. In this manner the difficulties of stratified charge burning are not present. The bulk of the fuel is burned at rapid velocity in the initial phase of combustion. Then, when the mixture of unburned fuel and very hot exhaust products mix with the remaining air the entire amount is at a temperature high enough to complete the combustion process rapidly.
A system that could be called xe2x80x98separated chargexe2x80x99 has been employed in versions of compression-ignition (Diesel) engine. Ricardo shows some varieties of this. The xe2x80x98pre-combustion chamber designxe2x80x99 and the xe2x80x98Comet Mark IIIxe2x80x99 can each be considered to utilize a combustion method that can be characterized as xe2x80x98separated chargexe2x80x99. In these engine configurations fuel is injected into a volume separated, by a short gas passage or passages, from the main cylinder volume. In this volume about 50% of the total air used by the engine is reduced by burning of the injected fuel in a manner that can be considered conventional compression-ignition engine spray combustion. Subsequently the hot mixture of fuel and combustion products are combined with the remaining air in the rest of the cylinder volume. The process allows up to 90% of the air to be burned (in the Comet Mark II) at full throttle showing that the process can be used to burn fuel at any level of leanness as long as all the fuel and some air are mixed in a fuel-rich burning amalgam in the initial phase of the burning process.